Unable to access services via DNS on RKE

Hello everyone,

we have a RKE-Cluster running on On-premise-Server and Cloud-Server connected via an OpenVPN-network.
Accoding to the Kubernetes-documentation we should be able to querry our services via DNS, but everytime we try to access our webapplications with curl it says something like “curl: (6) Could not resolve host: my-webapp.webtest.svc.cluster.local” , eventhough it seems that the CoreDNS is working well.

OS of the nodes: Ubuntu 18.04
RKE-Version: v1.0.10
Kubernetes-Version: v1.17.6
CoreDNS-Version: 1.6.5

Can someone please help me with this issue?